Non-destructive evaluation (NDE) techniques of optical video-based speckle interferometry are gaining importance as inspection tools, particularly by the aerospace industry [1,2]. An optical technique such as shearography is attractive to the NDE community largely because of its non-contacting nature, full-field measurement and fast inspection results. However, in order for this optical interferometric method to become widely used as an NDE tool, this technique must be made to be robust enough to operate in noisy environments typically found in industry settings. In this paper, we address these issues for the case of detection of disbonds using shearography in conjunction with synchronized pressure stressing

Chatters, Tom

Krishnaswamy, Sridhar

Pouet, Bruno

English

Non-destructive evaluation (NDE) techniques of optical video-based speckle interferometry are gaining importance as inspection tools, particularly by the aerospace industry [1,2]. An optical technique such as shearography is attractive to the NDE community largely because of its non-contacting nature, full-field measurement and fast inspection results. However, in order for this optical interferometric method to become widely used as an NDE tool, this technique must be made to be robust enough to operate in noisy environments typically found in industry settings. In this paper, we address these issues for the case of detection of disbonds using shearography in conjunction with synchronized pressure stressing.</p

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SHEAROGRAPHY WITH SYNCHRONIZED PRESSURE STRESSING Tom Chatters Bruno Pouet Sridhar Krishnaswamy Center for Quality Engineering and Failure Prevention Northwestern University Evanston, IL, 60208 INTRODUCTION Non-destructive evaluation (NDE) techniques of optical video-based speckle interferometry are gaining importance as inspection tools, particularly by the aerospace industry [1,2]. An optical technique such as shearography is attractive to the NDE community largely because of its non-contacting nature, full-field measurement and fast inspection results. However, in order for this optical interferometric method to become widely used as an NDE tool, this technique must be made to be robust enough to operate in noisy environments typically found in industry settings. In this paper, we address these issues for the case of detection of disbonds using shearography in conjunction with synchronized pressure stressing. Shearographic interferometry provides full-field, real-time information about the gradient of deformation of the test object. Instead of recording the interference between a reference beam and an object beam as in ESPI, interference is generated between spatially shifted (sheared) duplicate images of the test object surface. The direction of shearing of the images detennines the gradient direction. The degree of sensitivity is dependent on the amount of shearing induced hetween the two duplicate images [3]. An interference pattern of the test object surface in the un deformed state is compared to an interference pattern of the surface in the deformed state. Vmious processing methods including analog or digital signal processing techniques can he used to extract the desired information about the relative object deformation [4,5]. This infonnation is typically displayed in the form of an interference fringe pattern. For NDE applications, the method of stressing of the test object is suitahly chosen so as to cause internal structural defects in the object to manifest themselves on the surface of the test object as regions of strain non-uniformity. It is clear from the foregoing description, that shearography can be quite susceptible to extraneous noise. In pmticular, the technique is affected by any noise that is induced by slow object drift or thelmal CUlTents that occur dming the time of recording of each speckle pattern and, more importantly, between the time of recording of the pair of speckle patterns that are to he compared for relative displacement gradient infOlmation. Often times, the optical phase changes cOlTesponding to environmental noise are larger than the phase changes related to the relative ohject displacement infOlmation, possibly leading to the complete suhmerging of the signal hy the noise. Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New York, 1993 419 Recently, we described a reference-updating technique combined with synchronized acoustic stressing which minimizes the noise susceptibility of the conventional techniques [6]. In this paper, we descl1be a similar reference-updating technique for shearography in conjunction with synchronized pressure stressing for the detection of disbonds in adhesively bonded specimens. The proposed technique is compared with classical shearography in terms of optical noise reduction, and fringe stability and contrast. Conventional and synchronized reference-updating shearography are compared by purposely introducing noise into the expel1mental setup. In this comparison, a t1at-bottom hole specimen is used. Finally, realistic adhesively bonded specimens are investigated for disbonds using the reference-updating technique combined with synchronized pressure stressing, demonstrating its practical application in non-destructive evaluation. PRINCIPLE OF PROPOSED TECHNIQUE A schematic of the shearography setup is shown in Figure I (the pressure stressing device is not shown). A coherent laser beam is passed through an acousto-optic modulator which is used as an optical shutter. In shearography, only a single laser beam is required to illuminate the object. Duplicate images of the object surface are generated using a beamsplitter and two minors. The interference of the duplicate images of the test object results in an interference speckle pattem at the image plane of a CCD camera for recording into a digital image processing computer system. A transparent vacuum chamber (described below) is mounted on the disbond specimen and vacuum is applied to the specimen. The idea behind pressure stressing is to apply a pressure differential that would cause an intemal disbond to bulge out in a manner different from the surrounding bonded region. mirror 420 minor U,·ou<to·optic lI1odu]"tor laser r\·.-----~bcUm - -----------};I ima~e procc.."sor diffuse specimen computer l----- --- ------l::l-Q Figure 1 - Shearographic Interferometer. Image Processor Computer System and Display CCD Video Signal SynclU'onization System Transparent PMMAPlate -- DJuminatiuIl DirectioIl ----... ~_ViewingDirectiOn----Vacuum Source Figure 2 - Pressure Synchronization System. In a departure from conventional shearography, in the proposed technique, the pressure stressing system, the optical shutter and the CCD image acquisition system are all synchronized through an electronic synchronization system shown schematically in figure 2. The imaging CCD camera outputs a composite signal that contains the analog video image superposed on synchronization signals demarcating image frames (vertical sync). For US standard RS-170 cameras, each frame is acquired in one-thirtieth of a second. The synchronization electronics strips the vertical sync signal from the composite video and uses that as the time base for all the other subsystems in the setup. The key to the noise reduction techniques proposed in this work is to repetitively stress the test object between the reference and test states, and to do so in synchronization with the video imaging system. Furthermore, the laser illumination is also shuttered by the acousto-optic modulator that can be synchronized to the stressing cycles. The timing sequence aligns an illumination pulse with the unstressed or minimum of tlie stressing cycle (reference state) during one frame, and another illumination pulse with the peak of the stressing cycle (test state) during the next frame (see figure 3). CCD Image Acquisition Stressing Control Illumination Pulsing Vacuum Response Figure 3 - Pressure Stressing Timing Sequence. 421 1HEORETICAL DESCRIPTION In general, the theoretical descIiption of interference flinges resulting from the processing of two sheared-speckle interference images is well documented [3,7]. The CCD camera acquires a series of speckle images into the image processor at video rates. Since the CCD camera integrates the intensity of light incident on it over the duration of a frame peliod, the intensity recorded at any point in the image plane of the CCD camera duIing the recording time T of the nth frame may be expressed as (1) Here En(t) is an optical shuttering function of the acousto-optic modulator and 't' is the time from the start of the frame which is re-initialized to zero at the statt of every frame. The reference and object beam amplitudes are denoted by AR and Ao which are assumed to be essentially time invaliant dming the CCD integration time of anyone frame, but could, in general, vary from one frame to another due to low frequency noise. The quantity (2) is the difference in the phases of the reference and the object beams, and consists of a time-invariant (again, within one frame) random speckle phase component denoted for simplicity by qln , and a possibly time-varying part Md(t) which represents the signal of interest and can be related to the gradient of out-of-plane displacement of the object. In subtractive shearography, the signal Md(t) representing the object deformation can be extracted from the speckle images by subtracting one image (say, the CUlTent n 1 th frame) from a previously recorded and stored image (say, the reference noth frame). Thus, the quantity (3) is computed digitally and displayed on a video monitor in real-time. In order for the relative displacement gradient information Md(t) to be visible, it is essential that the speckle patterns corresponding to the current (test state) and reference images stay cOlTelated. That is, it is required that the reference and object beam amplitudes AR and Ao, as well as the random phase term qln be the same for both the current n1 th frame and the reference noth frame. In conventional shearography, the reference image is typically acquired at the beginning of a test with the specimen in the undefOlmed state (i.e. no=l, the 1irst frame), and the current image is then subtracted from this reference as the test object is stressed. It is clear that in an industrial environment where thelmal noise and slow object drift are present, the assumption that the speckle patterns stay con'elated will be increasingly violated as the time between the CUlTent image and the acquired referencc image becomes large. It is for this reason that fIinge stability and contrast degrade with time in conventional shearography, requiIing that a new reference image be acquired before the test can be resumed. In the proposed technique, this prohlem is overcome hy compating the current image with the il11l11l'diatl'iy preceding acquired image which serves as the reference image (i.e. no=nl-l). This process is repeated in real-time for every pair of images. It is reasonahle to expect that the speckle images will stay con'elated for two successive video frames if the amhient noise is of sufficiently low frequency compared to the video acquisition rate. Thus, flinge contrast is enhanced and flinge stability is assured with the synchronized reference-updating scheme. However, for the proposed technique to work, the deformation information contained in Md(t) that is recorded in consecutive frames must be different. That is, the object must he rapidly and repetitively defOlmed from one state to another in synchronization with the image acquisition sequence. This is achieved by use of 422 a transparent low-volume two-p0l1 vacuum chamber such that the specimen on which it is mounted can be illuminated and viewed. One of the POlts is connected to a vacuum source (pump and accumulator). The other port is connected to the ambient via a small solenoid-actuated valve which is used to cycle the vacuum level inside the chamber synchronously with the CCD camera. The timing sequence for pressure stressing is shown in figure 3. Each trough and crest in the first trace represents one frame of the image acquisition series. The signals sent to the solenoid valve during the (n-l)th and nth frames are given by (4) where H(t) is a step function stat1ing at time zero of each frame. Stressing is therefore applied only in every alternate frame and is of stepwise duration stal1ing at t 1 and ending at t2 from start of the frame. In view of the response time of the vacuum system, however, the actual pressure variation in the chamber is somewhat different as shown schematically in figure 3. Thus, the reference state should he acquired only after the valve has been opened awhile, and the next defOimed state should be acquired only after the valve has been closed awhile. This is achieved by use of the optical shutter (see figure 3) which is so chosen that only infOimation regarding the maximum defOlmed state is obtained during one frame, while only information regarding the relaxed undefonned state is acquired during the next frame. The optical shutter functions for the (n-l )th and nth frames are therefore given by (5) which represents a single illumination pulse being applied in each frame at time t2 from the start of the frame. Substituting the stressing function (4) and the optical shutter function (5) in (3), the video monitor signal, V s, becomes (6) These are the conventional sinusoidal fringes expected from shearography except that here fringe stability and contrast are improved due to continual reference-updating, which prevents noise from accumulating. RESULTS Specimens werc fahticated to simulate adhesivc disbonding that occurs in lap joints and honeycomb composite structures. The specimen fabrication was done using composite manufactming techniques as specified by the adhesive manufacturer. The specimens consist of two aluminum plates which are bonded together using a sheet of 3M aircraft epoxy AF-163-2K structural adhesive especially developed for bonding of aluminum. The top plate of the specimen is thin, while the bottom plate is relatively thick in order to provide a stiff SUpp0l1 to the entire specimen. Artificial disbonds are made by introducing Tenon insel1s betwcen the epoxy sheet and the top plate to prevent bonding in selected areas of the two aluminum plates. The Tenon insel1s are sandwiches of the desired shape made from two 0.001" thick sheets of Tenon which are heat-fused together around the edges. The specimens are cured at a temperature of 2500 F for 1 hour under a pressure of 40 psi, as stipulated by the adhesive manufacturer. Disbond specimens have been fablicated with 1/32, 1/16 and 1/8 inch thick front plates coveling artiticial dis bonds of 3, 2, 1, 1/2 and 114 inch diameters bonded to 112 inch thick back plates. The viewing area of the specimens was 12 inches square. In addition, a nat-bottom hole specimen was fabricated by milling a 3 inch diameter hole, 31/32 inch in depth, in a 1 inch thick aluminum plate to create a t1exihle memhrane with a relatively stiff supporting structure. 423 For all the experiments, specimens underwent repetitive pressure stressing at 15 Hz. The level of vacuum differential obtained was approximately l/lOth of an atmosphere. For purposes of comparison, two images of the nat-bottom hole specimen were acquired with conventional and synchronized reference-updating pressure stressing methods using shearography. Slow object drift was induced in the specimen mounting by using a screw-driven mechanism to generate decoo-elating motion of the specimen. This was an attempt to simulate noise in practical NDE situations. Figures 4a, band c show a time sequence of typical real-time shearographic images recorded using the conventional pressure stressing setup. As time progresses (from a to c) after acquiring the reference image, the fringes using the conventional technique continues to degrade and eventually disappear due to increasing speckle decorrelation. The corresponding result for the synchronized reference-updating scheme is shown in figure 4d. Although it cannot be shown in graphic still images, the video display of the synchronized technique in action shows significant improvement in image stability with no degradation with time in image contrast because noise-induced speckle decOlTelation does not build up in this case. This is of significance 424 a b c d Figure 4 a-c - Sequence of Conventional Shearography Fringe Patterns; and d :Reference-Updating Shearography Fringe Pattern. in real-time NDE applications since defect signature stability and contrast would improve probability of detection by the operator. Next, the issue of defect detectability was addressed using the synchronized pressure stressing method by investigating specimens containing disbonds of varying diameters and depths. In figures Sa and b are shown the shearography fringe patterns corresponding to disbonds of varying sizes and at depths of 1/32 and 1116 inch, respectively. Here, the CCD camera was focused on each disbond in order to better resolve the fringes . The amount of the disbond deformation depends on the disbond size (larger disbond diameters deform more for fixed plate thickness). Figure 6 shows an overview of the plate under pressure stressing using synchronized shearography. In this image, four dis bonds of various geometry are visible simultaneously. It should be pointed out that in all these experiments, unlike conventional shearography where speckle decorrelation invmiably occurs sooner or later, the flinge pattems obtained with tbe synchronized reference updating technique were very stahle. As an added advantage, this Figures Sa and h - 1" Dia. Dishond and 2" Dia. Dishond. Figure 6 - Overview of Four Dishonds. 425 method, with its constant synchronized renewal of the reference, also automatically recovers from any momentary loss of information caused by sudden gross object motion. CONCLUSION In this work, reduction of noise-sensitivity of shearographic NDE is achieved using a reference-updating technique combined with synchronized pressure stressing. In this manner, low frequency speckle decolTelating noise is eliminated. It is shown that the technique improves flinge stability as well as enhances flinge contrast. The concept of synchronized reference-updating is expected to enhance the ability of the optical NDE techniques to be used in the field where non-ideal environmental conditions exist. ACKNOWLEDGMENT This work was carried out at Northwestern University in the course of research sponsored by the FAA Center for Aviation Systems Reliability, operated by Ames Laboratory, USDOE, for the Federal Aviation Administration under Contract No. W-7405-ENG-82 for work at Iowa State University. REFERENCES 1. D.D. Burleigh, Shearographic NDT of Foam Insulation for Cryogenic Fuel Tanks, Spring Conference of the Amelican Society for Nondestructive Testing, Orlando, FIOlida (1992). 2. J.F. Clarady, Electronic Holography and Shearography NDEfor Inspection of Modern Materials and Structures, Spling Conference of the Amelican Society for Nondestructive Testing, Orlando, Florida (1992). 3. Y.Y Hung, Shearography: A Novel ({nd Practical Approach for Non-Destructive In.I]Jection, Journal of Non-Destructive Evaluation, Vol. 8, No.2., p.55 (1989). 4. 0.1. Lokberg, O.M. Holje and H.M. Pedersen, Scan Converter Memory Used in TV-Speckle Inteiferometry, Opt. Laser Techno!., Vo!. 8, p.17 (1976). 5. S. Nakadate, T. Yatagai, and H. Saito, Electronic Speckle Pattern lnteiferometry Using Digital Image Processing Techniques, App!. Opt. Vol. 19, p.1879 (1980). 6. B.F. Pouet, T.C. Chatters, and S. Klishnaswamy, Synchronized Reference-Updating Technique.f(Jr Electronic Speckle Intelferometly (submitted to the Journal of Nondestructive Testing) (1992). 7. F.S. Chau, H.M. Shang, S.L. Toh, and c.J. Tay, Some Examples of Non-Destructive Flaw Detection by Shearography, Journal of Non-Destructive Evaluation, Vo!. 8, No.4, p. 225 (1989). 426 

Shearography with syncrhonized pressure stressing

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Shearography with syncrhonized pressure stressing

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